Steam flooding hydrocarbon recovery process

ABSTRACT

A method for recovering hydrocarbons from a subterranean formation in which steam, for example, is injected into the formation via an injection well to drive the hydrocarbons toward a spaced production well. Water injected via the production well into an upper horizon of the formation is passed downwardly through the formation about the production well and then back into the production well at a lower horizon in the formation. A mixture of displaced hydrocarbons and injected water are then coproduced at the lower horizon thru the production well after which the hydrocarbons are recovered from the mixture. Optionally, a minor amount of a sulfated interfacial tension reducer may be introduced with the steam to promote increased hydrocarbon recovery.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a steam flooding process for the recovery ofhydrocarbons from a subterranean formation. More particularly, thisinvention relates to a recovery process in which hydrocarbons driventoward a production well by means of a fluid, such as steam or a mixtureof steam and carbon dioxide, which is injected into the formation via aninjection well are coproduced from a lower horizon of the formation viathe production well along with water introduced into the upper horizonof the formation.

2. Prior Art and Background

The production of hydrocarbon products is usually achieved by drillinginto a hydrocarbon-bearing formation and employing one of theart-recognized recovery methods for the recovery of hydrocarbonstherein. Present recovery techniques, however, usually result in therecovery of only a minor portion of the petroleum materials present inthe formation and this is particularly true with reservoirs of viscouscrudes. Even when employing improved secondary recovery practices asmuch as 50-75 percent of the original hydrocarbon may be left in placeand even more in the case of viscous hydrocarbon reservoirs.

A variety of processes well known in the art, such as water flooding,steam flooding, miscible flooding, etc. have been employed after naturaldrive of the reservoir has been depleted in order to recover additionaloil from the formation. The application of these techniques which aresometimes referred to as secondary recovery methods permits additionalhydrocarbons to be removed from the partially depleted formations. Oneof the more widely practiced secondary recovery methods is that of theso-called steam flooding process. Steam flooding is notably well-suitedfor secondary recovery operations since the energy contained in thefluid effectively reduces the viscosity of the hydrocarbons and permitsproduction thereof. In order to realize the maximum viscosity reductionof the hydrocarbons, the injected steam should impart the maximum heatto the formation, as is consistent with economical steam generatordesign, and provide a uniform penetration of the formation.

Despite the advantages of steam flooding operations, under certaincircumstances present-day steam flooding techniques fail in manyinstances to permit recovery of large quantities of hydrocarbonscontained in the formation. As a result, a number of modified steaminjection processes have been proposed including a "push-pull" techniqueand throughput methods which have resulted in some instances inadditional significant recoveries of crude oil from the reservoirs.

One of the main problems faced in the recovery of hydrocarbons by steamflooding is early breakthrough of steam into the production well sinceat that time no more oil is produced. Breakthrough generally is causedby the tendency of the steam to move updip and to flow only through theupper part of the formation.

There is a definite need in the art, therefore, for a steam floodingprocess in which steam breakthrough into the production well isprevented thus greatly increasing the amount of oil recovered.

Another disadvantage of steam flooding is that some distillation in theformation takes place with the result that lighter, more volatilesolutions of the in-place hydrocarbons are recovered leaving behind themore viscous oil with an increased asphaltene and aromatic content.Thus, the nature of the residual oil left behind after an initial periodof steam flooding of a formation is probably different from that of theoriginal oil composition in place because of distillation effects, etc.and the efficiency of the removal process gradually declines.

Accordingly, it is an object of the present invention to overcome thedisadvantages of the prior art steam flooding process by providing anefficient, improved steam flooding method for hydrocarbon recovery.

BRIEF SUMMARY OF THE INVENTION

This invention provides an improved method of recovering hydrocarbonsfrom a subterranean hydrocarbon bearing formation wherein the formationis penetrated by an injection well and a production well whichcomprises:

a. injecting steam or a mixture of steam and carbon dioxide via theinjection well into the said hydrocarbon-bearing formation to drive saidhydrocarbons toward said production well,

b. injecting water via the production well into the surroundingformation at an upper horizon of the said hydrocarbon bearing formationand passing said water downwardly through the formation about theproduction well to a lower horizon in the said formation and then backinto the production well from the surrounding formation at the lowerhorizon,

c. producing a mixture of the displaced hydrocarbons and the injectedwater at the lower horizon via the said production well and recoveringhydrocarbons from said mixture.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE schematically illustrates a method in accordance with thisinvention in which hydrocarbons are recovered by steam flooding and inwhich steam breakthrough into the production well is prevented.

It is known that steam does not displace reservoir oil with goodvolumetric efficiency due to one or a combination of the flowingreasons:

1. Gravity segragation due to the low density of steam as compared tocondensate, oil and interstitial water;

2. Whenever gas saturation and gas permeability exists, steampreferentially displaces the in place gas and flows to producing wellsvia the gas channels present at the initiation of steam injection;

3. For formations having a high degree of permeability distribution, thesteam channels to producing wells via the higher permeability strata.

In practice the steam front as it advances from the injection welltoward the production well becomes tilted. As a result, earlybreakthrough of steam is experienced with no more oil production fromthe well.

The above remarks apply to reservors having high vertical permeability.For zones having an appreciable degree of vertical permeabilitydistribution, steam can channel through the most permeable path or pathsvery early and result in low volumetric oil displacment efficiency.

Whenever a gas saturation exists prior to steam injection, the steamdisplaces the in place gas first. If the gas saturation is above thecritical gas saturation (about 5%) gas permeability exists and ismaintained by gas (steam) displacing gas. As a result, the steam breaksthrough into producing wells with low displacement efficiency.

As previously pointed out whenever steam breaks through into a producingwell or wells no more oil is produced. Surprising, it has been foundthat it is possible to regain and maintain oil production by cooling theformation adjacent the well according to the process of this invention.Temperature reduction of the formation will condense all steam, reducethe gas saturation to zero by condensing the steam. The initial in placegas at this point, i.e., the formation adjacent the producing well, hasalready been produced and reduction of gas (steam) saturation to zerosimultaneously reduces the gas (steam) permeability to zero with nosteam production.

Cooling the formation adjacent the producing well can be accomplished asset out in the process of this invention by single well bore cyclingusing water as the cycle fluid. In this process water is injected intothe top of the zone while displaced hydrocarbons and injected water areproduced from near the bottom of the zone, each interval being separatedby a packer. Preferably, in this process injection and production ratesare adjusted so that all steam adjacent the well bore is condensed andproduction contains cycle fluid, i.e., water and displaced hydrocarbons.Production profile surveys are utilized for determining the interval orintervals flowing to a well bore and to determine the section in whichsteam is flowing, a temperature survey is conducted.

If desired, the steam injected via the injection well may contain fromabout 0.01 to about 0.10 weight percent or more of an interfacialtension reducer in order to increase the oil recovery. Alkaline fluidsmay also be injected via the injection well in the process of thisinvention. The injection fluids are made alkaline by the addition ofsodium hydroxide or potassium hydroxide to the steam in an amountsufficient to give a concentration of about 0.01 to about 0.1 weightpercent based on the total fluid weight.

Interfacial tension reducers which are highly useful in the process ofthe invention include sulfated compounds of the formula: ##STR1##wherein r is an integer of from 2 to about 5, s is an integer of from 8to about 60, wherein the sum of r plus s is not more than 55 and M isselected from the group consisting of hydrogen, sodium, potassium, andthe ammonium ion and compounds of the formula: ##STR2## wherein t is aninteger of from 8 to about 40, and M has the same meaning as previouslydescribed.

Interfacial tension reducers of this type can be formed by sulfatingcompounds of the formula: ##STR3## and where r and s have the samemeaning as before and compounds of the formula: ##STR4## where t has thesame meaning as before, batchwise with chlorosulfonic acid in a glasslined kettle at about 30° C. followed by reaction with the correspondingbase, if desired. The corresponding starting compounds (II) can beprepared in the same manner as described in U.S. Pat. No. 3,731,741employing as starting aromatic compounds 8-quinolinesulfonyl chloride,6-quinolinesulfonyl bromide, etc., as initiators and reacting theinitiator first with the necessary amount of propylene glycol of therequired molecular weight followed by the necessary amount of ethyleneglycol of the required molecular weight. The quinoline starting materialmay also be substituted by other innocous groups such as alkoxy of from1 to 4 carbon atoms, alkyl, etc.

In the FIGURE a hydrocarbon-bearing formation is shown together with asuitable apparatus for conducting the recovery method of this invention.It should be noted that other formations and other arrangements ofapparatus may be utilized in carrying out the steps of this method. Inthe FIGURE there is shown a hydrocarbon-bearing formation 20 situatedbelow the earth's surface 22 covered by overburden 24 and supported bystrata 26.

Formation 20 is penetrated by spaced-apart injection well 28 andproduction well 30. Injection well 28 is equipped with casing 32 and isprovided with perforations 34 within the lower part ofhydrocarbon-bearing formation 20. The bottom of casing 28 is sealed bymeans of casing shoe 36 and wellhead 38 encloses the top of casing 32.Injection well 28 is equipped with tubing 40 which extends downward fromwellhead 38 through packer 42 located just above the uppermost ofperforations 34 and tubing 40 terminates adjacent the said perforations34. Inlet steam line 44 is connected to tubing 40 at wellhead 38. Duringthe operation of this process steam which is introduced into tubing 40via steam inlet line 44 passes into the producing formation throughperforations 34.

Production well 30 is equipped with casing 46 and is provided with upperperforations 48 within the upper part of hydrocarbon-bearing formation20 and lower perforations 50 within the lower part ofhydrocarbon-bearing formation 20. The bottom of casing 46 is sealed bymeans of casing shoe 52 and wellhead 54 encloses the top of casing 46.Production well 30 is equipped with tubing 56 which extends downwardfrom wellhead 54 through packer 58 which is placed in casing 46 betweenupper perforations 48 and lower perforations 50 thus dividing theproduction well 30 into two separate fluid handling areas, i.e., anupper area and a lower area. Tubing 56 terminates adjacent the lowerperforations 50. Production well 30 is provided with water inlet line 60which passes through wellhead 54 into the annulus section between tubing56 and casing 46 and above packer 58. During operation of this processwater is introduced from the earth's surface via line 60, down theannulus space above packer 58; i.e., the upper fluid handling area, andthen out into the formation through upper perforations 48. The waterthus introduced passes downwardly through the formation about theproduction well and simultaneously contacts and condenses any steamoriginally introduced into the formation via the injection well whichbreaks through and arrives in the vicinity of the production well. Theresulting fluid which includes hydrocarbons displaced through theformation is produced from the lower horizon of the surroundinghydrocarbon-bearing formation 20 through the lower perforations 50 intoproduction well 30. The produced fluid is conducted to the earth'ssurface 22 through tubing 56 and finally is conveyed via line 62 to asuitable hydrocarbon recovery facility.

The following example illustrates one embodiment of this invention andis to be considered not limitative.

EXAMPLE I

An oil reservoir in South Texas has the following properties.

    ______________________________________                                        Depth                 2,000 ft.                                               Thickness             50 ft.                                                  Porosity              35%                                                     Permeability                                                                   Horizontal           3.0 darcies                                              Vertical             2.5 darcies                                             Initial Saturation                                                             Oil                  80%                                                      Water                20%                                                     Oil Gravity           14° API                                          ______________________________________                                    

The above-described formation is produced by solution gas drive yieldingonly 10% of the in-place oil and resulting in the following saturations:

Oil: 72%

Gas: 8%

Water: 20%

The field is then converted to a steam flood utilizing a five-spotpattern basis with a central injection well and with the producing wells750 feet apart. Due to good vertical permeability the injected steammigrates to the top of the reservoir where it breaks through into theproducing wells. There is some productivity increase shortly after steaminjection is initiated, the average rate increasing from about 5 to 50barrels of oil per day (i.e., BOPD) per well. However, after about onemonth's operation steam break through is experienced and the productionis reduced to only 2 BOPD per well. During this initial short period ofwell stimulation less than 0.1% of the in place oil is recovered.

A temperature survey using vertically situated thermocouples shows thatsteam broke through the producing wells near the top of the pay. Theproduction wells are then completed as shown in the FIGURE. The wellsare produced through the tubing, by flow and/or pump, and water isinjected down the annulus between casing and tubing. Steam and 515 psia,470° F. (quality = 80%) is injected into the formation via the tubingand casing perforations of the injection well. After adjusting, theinjection and the producing rates are stabilized at 100 barrels of waterper day injection, with production of 300 barrels of total fluid per daycontaining 150 barrels of oil, 100 barrels of injected water and 50barrels of water from the formation. The bottom hole flowing temperatureis below steam temperature for the pressure at that location and nosteam flows to the well bore. The overall rate is maintained until thenet oil production becomes uneconomical. After this condition developsin all producing wells the steam flood is abandoned. Oil recoveredduring steam flooding is about 60% of the original oil in place bringingthe total recovery to about 70%.

In certain fields steam breakthrough may occur in the production wellsnear the bottom of the oil zone. In such cases water is injected throughthe tubing to a point below the packer set between the upper and lowercasing perforations and production is through the upper casingperforations via the annulus. If the well will not flow, it may benecessary to use a cross-over packer for pumping the well whileinjecting water.

What is claimed is:
 1. A method for recovering hydrocarbons from a subterranean, hydrocarbon-bearing formation penetrated by an injection well and a production well which comprises:a. injecting via the injection well a fluid selected from the group consisting of steam and a mixture of steam and carbon dioxide into the said hydrocarbon-bearing formation to drive said hydrocarbons toward said production well, b. injecting water via the production well into the surrounding formation at an upper horizon of the said hydrocarbon-bearing formation, and passing said water downwardly through the formation about the production well to a lower horizon in the said formation and then back into the production well from the surrounding formation at the lower horizon, c. producing a mixture of the displaced hydrocarbons and the injected water at the lower horizon via the mixture, and wherein the said fluid injected in (a) contains from about 0.01 to about 0.10 weight percent of an interfacial tension reducer selected from the group consisting of a compound of the formula: ##STR5## wherein r is an integer of from 2 to about 5, s is an integer of from 8 to about 60, wherein the sum of r plus s is not more than 55 and M is selected from the group consisting of hydrogen, sodium, potassium and the ammonium ion, and a compound of the formula: ##STR6## wherein t is an integer of from 8 to about 40, and M has the same meaning as previously described.
 2. The method of claim 1 wherein the said fluid injected in (a) is steam.
 3. The method of claim 1 wherein the said fluid is steam and carbon dioxide containing from about 5 to about 40 percent by volume of carbon dioxide. 